1) Field of the Invention
The present invention relates to a process for the preparation of zeolites in which some of the silicon is substituted with titanium.
2) Background Art
The zeolites of type MFI in which some of the silicon atoms are substituted with titanium atoms are known products which are described, in particular, in the U.S. Pat. No. 4,410,501; these zeolites are designated TS-1. In accordance with the abovementioned U.S. Patent the zeolites TS-1 are prepared by reaction in the aqueous phase at between 130.degree. and 200.degree. C. and under autogenous pressure of a silicon source (for example alkyl orthosilicates, colloidal silica, alkali metal silicates) with a titanium oxide source consisting of a hydrolyzable derivative of titanium (TiCl.sub.2, TiOCl.sub.2, alkyl titanates) in the presence of a nitrogenous base such as a quaternary ammonium hydroxide, in the absence of alkali metal ions. In this process, the preferred sources of silicon and of titanium which have been recommended are tetraethyl silicate and tetraethyl titanate; tetrapropylammonium hydroxide (TPAOH) represents the base which is most suitable for the implementation of the process. The zeolites TS-1 obtained by this process conform to the general formula: EQU xTiO.sub.2 (1-x)SiO.sub.2
in which x is an integral or fractional number which is greater than 0 and less than or equal to 0.025.
Zeolites of type MEL in which some of the silicon is replaced by titanium (zeolites TS-2) have been prepared by an analogous process, using tetrabutylammonium hydroxide as structuring agent (cf. J. S. Reddy et al., Appl. Catal. 58, page 1 (1990)).
These titanosilicalites of type MFI or MEL have rapidly shown themselves to be excellent catalysts in a large number of reactions in organic chemistry, and in particular for the oxidation of organic compounds using hydrogen peroxide. However, it has been found that the catalytic activity of the zeolites TS-1 is affected by the presence of extrareticular titanium (cf. U. Romano et al., Chim. Ind. (Milan) 72, p.610 (1990); T. Tatsumi et al., J. Chem. Soc. Chem. Commun., p.476 (1990)). This presence results from the precipitation of TiO.sub.2 by premature hydrolysis of the titanium source under the reaction conditions, especially when use is made of tetraethyl orthotitanate, which very readily undergoes hydrolysis at room temperature. In order to avoid this precipitation and to increase the content of intrareticular titanium in the titanosilicalites, A. Thangaraj et al., J.Catal. (1991) vol.130 p.1-8; ZEOLITHES vol.12 p.943 (1992) have proposed the replacement of tetraethyl orthotitanate by a less hydrolytable compound; to this effect they have recommended the use of tetrabutyl titanate in solution in anhydrous isopropyl alcohol and of specific procedures for employing the reactants. According to these authors the molar Si/Ti ratio of the titanosilicalites thus obtained would have been lowered to a value in the region of 10, a result which is, moreover, contested by some authors (cf. R. Millini et al., J.Catalysis 137, p.497 (1992)).
A further disadvantage of the abovementioned processes lies in the need to use structuring agents which are free of alkali metal or alkaline earth metal ions, in order to avoid the formation of alkali metal titanates or alkaline earth metal titanates, which are inactive in the reaction of formation of the zeolite. This involves treatment of the commercial aqueous solutions of quaternary ammonium hydroxides prior to their use, for example by electrodialysis.
To sum up, the processes which have been used to date possess disadvantages, which complicate their industrial implementation, and there exists a need to provide industry with a process which is not subject to the constraints of using an anhydrous carrier for the titanium source and of removing the alkali metal ions from the structuring agent.
The present invention proposes to provide industry with a process for the manufacture of zeolites containing titanium, which does not have the drawbacks of the prior processes.
To this end a first objective of the present invention is to do away with the need to use an anhydrous carrier in order to add the titanium source to the reaction medium. A second objective of the present invention is to avoid the rapid hydrolysis of the titanium source when it is placed in contact with the structuring agent. A third objective is to develop a process which does not require the use of a quaternary ammonium hydroxide which is free of alkali metal ions.